In an exhaust system of an internal combustion engine which performs lean combustion like a diesel engine, a NOx storage/reduction catalyst device is arranged. The NOx storage/reduction catalyst device satisfactorily stores NOx when the air-fuel ratio of the exhaust gas is lean, namely when the concentration of oxygen in the exhaust gas is high. On the other hand, as a regeneration treatment, when the air-fuel ratio of the exhaust gas is made a stoichiometric air-fuel ratio or a rich air-fuel ratio, namely, the concentration of oxygen is decreased, the NOx storage/reduction catalyst device releases the stored NOx and purifies to reduce the released NOx by reducing materials in the exhaust gas.
The exhaust gas of the internal combustion engine also includes particulates. It is desired to trap the particulates and reduce the amount of particulate emitted to the atmosphere. Accordingly, it is suggested that a particulate filter for this purpose is arranged downstream of the NOx storage/reduction catalyst device.
The particulate filter requires a regeneration treatment to burn off the trapped particulate before the trapped particulate causes a large exhaust resistance. As the regeneration treatment, it is suggested that additional fuel be supplied to the exhaust gas, a part of the additional fuel is burned in the NOx storage/reduction catalyst device to increase the temperature of the exhaust gas, the relatively high temperature exhaust gas and the remaining additional fuel flow into the particulate filter, the remaining additional fuel is burned in the particulate filter the temperature of which is increased by the relatively high temperature exhaust gas, and thus the temperature of the particulate filter is further increased to burn off the trapped particulate (for example, refer to Japanese Unexamined Patent Publication No. 2006-291825).